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Concept
Iodometry
Summary
Iodometry, known as iodometric titration, is a method of volumetric chemical analysis, a redox titration where the appearance or disappearance of elementary iodine indicates the end point.
Note that iodometry involves indirect titration of iodine liberated by reaction with the analyte, whereas iodimetry involves direct titration using iodine as the titrant.
Redox titration using sodium thiosulphate, (usually) as a reducing agent is known as iodometric titration since it is used specifically to titrate iodine. The iodometric titration is a general method to determine the concentration of an oxidising agent in solution. In an iodometric titration, a starch solution is used as an indicator since it can absorb the that is released. This absorption will cause the solution to change its colour from deep blue to light yellow when titrated with standardised thiosulfate solution. This indicates the end point of the titration. Iodometry is commonly used to analyse the concentration of oxidizing agents in water samples, such as oxygen saturation in ecological studies or active chlorine in swimming pool water analysis.
To a known volume of sample, an excess but known amount of I- is added, which the oxidizing agent then oxidizes to I2. I2 dissolves in the iodide-containing solution to give triiodide ions (I3-), which have a dark brown color. The triiodide ion solution is then titrated against standard thiosulfate solution to give iodide again using starch indicator:
(E0 = +0.54 V)
Together with reduction potential of thiosulfate:
(E0 = +0.08 V)
The overall reaction is thus:
(Ereaction = +0.46 V)
For simplicity, the equations will usually be written in terms of aqueous molecular iodine rather than the triiodide ion, as the iodide ion did not participate in the reaction in terms of mole ratio analysis. The disappearance of the deep blue color is, due to the decomposition of the iodine-starch clathrate, marks the end point.
The reducing agent used does not necessarily need to be thiosulfate; stannous chloride, sulfites, sulfides, arsenic(III), and antimony(III) salts are commonly used alternatives at pH above 8.
Official source
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